README.rdoc in fluent-plugin-s3-0.3.0 vs README.rdoc in fluent-plugin-s3-0.3.1

@@ -29,12 +29,12 @@
       time_slice_format %Y%m%d-%H
       time_slice_wait 10m
       utc
     </match>
 
-[aws_key_id (required)] AWS access key id.
+[aws_key_id] AWS access key id. This parameter is required when your agent is not running on EC2 instance with an IAM Instance Profile.
 
-[aws_sec_key (required)] AWS secret key.
+[aws_sec_key] AWS secret key. This parameter is required when your agent is not running on EC2 instance with an IAM Instance Profile.
 
 [s3_bucket (required)] S3 bucket name.
 
 [s3_endpoint] s3 endpoint name. Example, Tokyo region is "s3-ap-northeast-1.amazonaws.com".